In recent years, as a storage apparatus for a computer, it is desired that one capable of writing at a high speed and unlimited number of times and also is a nonvolatile type.
And a magnetic storage apparatus using ferromagnetic tunnel junction devices is attracting attention as a storage apparatus which is provided with the above mentioned functions.
Such a ferromagnetic tunnel junction device is constituted by laminating a pair of ferromagnetic thin films on both the top and back surfaces of an insulation thin film. One of the ferromagnetic thin films is called a fixed magnetization layer because it is always magnetized in a fixed direction. Here, on the other hand, the other ferromagnetic thin film is called as a free magnetization layer because it is magnetized in the same direction (parallel direction) as the magnetization direction of the fixed magnetization layer or it is inverted in the opposite direction (anti-parallel direction) depending on the memory state of the ferromagnetic tunnel junction device. Moreover, the insulation is called a tunnel barrier layer because it causes a tunnel current when voltage is applied between the fixed magnetization layer and the free magnetization layer.
The ferromagnetic tunnel junction device has a structure which stably holds two states of different magnetization direction, that is, the state in which the free magnetization layer is magnetized in the same direction as that of the fixed magnetization layer or the state in which it is magnetized in the opposite magnetization direction to that of the fixed magnetization layer by the action of the magnetic force of the fixed magnetization layer, thereby storing the two states of different magnetization direction. By defining these two different states of magnetization direction as two different memory states, that is, “0” and “1”, it is possible to store two different memory states.
Accordingly, the ferromagnetic tunnel junction device is capable of writing two different memory states externally by magnetizing the free magnetization layer in the same magnetization direction as or the opposite magnetization direction to that of the fixed magnetization layer. It is to be noted that the memory states are written in the ferromagnetic tunnel junction device can be read out by utilizing the giant magneto resistance effect in which conductance in the tunnel barrier layer differs depending on the magnetization direction of the free magnetization layer.
Furthermore, the magnetic storage apparatus utilizing the ferromagnetic tunnel junction devices is formed with a plurality of first wiring lines on a semiconductor substrate in the magnetization direction of the fixed magnetization layers of the ferromagnetic tunnel junction devices, a plurality of second wiring lines on the semiconductor substrate in the orthogonal direction to the magnetization direction of the fixed magnetization layers of the ferromagnetic tunnel junction devices. The ferromagnetic tunnel junction devices are disposed at cross points of the first wiring and the second wiring, which are formed in a lattice-like pattern. Here, in conformity to conventional memory devices, such as DRAMs, SRAMs and the like, the first wiring is called a word line while the second wiring is called a bit line.
In the magnetic storage apparatus using ferromagnetic tunnel junction devices that is constituted as above, in order to perform a storage operation in the ferromagnetic tunnel junction device, a current is caused to flow through the word line so as to generate a word line magnetic force in the orthogonal direction to the direction of the current flow, and also a current is caused to flow through the bit line so as to generate a bit line magnetic force in the orthogonal direction to the direction of the current flow. As a result, a combined magnetic force of the word line magnetic force and the bit line magnetic force acts upon the free magnetization layer, and the free magnetization layer is then magnetized in the same direction as or the opposite direction to the magnetization direction of the fixed magnetization layer. In this manner, either one of the two states of magnetization direction generates in the free magnetization layer. Such state of magnetization direction is stably held by the action of the magnetic force of the fixed magnetization layer, thereby performing the storage operation in the ferromagnetic tunnel junction device.
Conventionally, in the case of writing a desired memory state in the ferromagnetic tunnel junction device, the current flowing through the word line is maintained always in the fixed direction so as to generate the word line magnetic force always in the fixed direction, and under this condition, only the bit line magnetic force is inverted by inverting the direction of current flowing through the bit line, thereby changing the magnetization direction of the combined magnetic force of the word line magnetic force and the bit line magnetic force. By the action of the combined magnetic force upon the free magnetization layer, the magnetization direction of the free magnetization layer is inverted, thereby writing a desired memory state in the ferromagnetic tunnel junction device.
However, in the aforementioned conventional magnetic storage apparatus using ferromagnetic tunnel junction devices, at the time of writing a desired memory state in the ferromagnetic tunnel junction device, the direction of the current flowing through the bit line is inverted while the direction of the current flowing through the word line remains always in the fixed direction, so that the current flows through the word line always in the fixed direction at the time of writing in the ferromagnetic tunnel junction device and, therefore, a constant potential difference is always generated between the word line and the peripheral area of the semiconductor substrate.
Moreover, in the magnetic storage apparatus, since it is necessary to generate a magnetic force by causing a current to flow through the word line, a current as large as several tens of milliamperes (mA) must be flowed through the word line.
Consequently, electromigration is occurs due to the current flowing through the word line in the fixed direction and potential difference generated between the word line and peripheral area thereof and separates out a metal or metals consisting of the word line, thereby causing a short-circuit between the word lines, damage of the word lines themselves, etc. Accordingly, there maybe a fear of incurring troubles in the magnetic storage apparatus.
A possible solution to prevent such electromigration from occurring is to improve the resistance to electromigration of the word lines by increasing the width of the word lines, however, which accompanies another problem of increasing the size of the magnetic storage apparatus due to the increased width of the word lines.
It is, therefore, an object of the present invention is to provide a magnetic storage apparatus with improved resistance to electromigration of the word lines without increasing the width of the word lines.